1. Field of the Invention
The present invention relates to a process for the recovery of uranium contained in an organic phase. More particularly, the invention concerns the concentration and purification of uranium extracted from a wet process phosphoric acid, i.e. from phosphoric acid produced by the acidulation of phosphate rock.
2. Discussion of the Prior Art
Phosphate rock naturally contains small amounts of uranium (from about 50 to 400 p.p.m.). The uranium is dissolved during acidulation of the phosphate rock and remains in the phosphoric acid solution thus produced. Although the concentration of urnaium in such solutions is low, the wet-process phosphoric acid is a valuable source of uranium because of the vast quantities of phosphate rock mined each year and processed to recover high-phosphate-containing fertilizer.
A number of attempts have been made in the past to develop commerically feasible methods of recovering uranium from such aqueous solutions. Thus, it is known to recover uranium from aqueous solutions in which it is present in low concentrations, by separating the other potentially usable components of the minerals treated by means of a liquid-liquid extraction assembly and by chemical treatment for the purpose of isolating the uranium and recovering it in the form of a high purity U.sub.3 O.sub.8 oxide, usable as a source of nuclear fuel. These processes are applicable to the recovery of uranium from minerals such as phosphate rock, which also yields phosphoric acid, and from minerals of various origin containing more or less uranium, which is present most frequently in the form of oxides. The prior art processes generally comprise treatment of the mineral with the aid of a strong and concentrated acid, such as sulfuric, phosphoric, hydrochloric or nitric acid, to provide an aqueous solution containing uranyl ions in a highly dilute state, together with other contaminating ions, from which the uranium is then recovered. A typical example of the treatment of such a solution from a raw, wet process phosphoric acid, obtained by the attack of surfuric acid on phosphate rocks, is described by F. J. Hurst & D. J. Crouse in Ind. Eng. Chem. Process Des. Develop. Vol. 11, No. 1, 1972, pp. 122-128. See also Hurst et al. U.S. Pat. No. 3,711,591 and Wiewiorowski et al. U.S. Pat. No. 3,737,513 for descriptions of various prior art attempts.
According to the Hurst and Crouse process described in the aforementioned Ind. Eng. Chem. Process Des. Development publication, the solution in which uranium is found or in which it is transformed to the U.sup.+6 state, is exposed to a first uranium extraction cycle employing an organic solvent consisting of a mixture of synergistic extracts, namely di-(2-ethylhexyl)-phosphoric acid (designated HDEHP or D2EHPA) and trioctylphosphine oxide (designated TOPO), diluted in a kerosene-type hydrocarbon. The uranium is extracted from the aqueous solution into the organic solvent in the form of a uranyl complex formed between the uranium (VI) UO.sub.2.sup.+2 ions and the synergistic mixture of the extractants. The uranium is subsequently recovered from the organic phase into which it has been extracted by means of contact with an aqueous solution of phosphoric acid containing sufficient iron (II) ions to reduce the uranium (VI) to uranium (IV). Because the quadrivalent state is less extractable, the uranium is transferred to the aqueous phase. This aqueous phase is then reoxidized to return the uranium to the uranium (VI) state of oxidation and is then exposed to a second extraction cycle using an organic phase containing a synergistic mixture of he HDEHP-TOPO extractants to obtain finally, after the re-extraction of uranium with an ammonium carbonate solution, a sufficiently pure mixed uranium and ammonium carbonate.
The prior art process described above has a number of disadvantages at the industrial level. Specifically, the reducing re-extraction in the first cycle requires the addition of iron (II) ions which are obtained by the action of phosphoric acid on iron, which is a slow and difficult reaction, or by means of the introduction of an iron (II) salt, which implies the introduction of an additional anion. In any case, this operating method has the disadvantage of introducing an appreciable amount of iron into the phosphoric acid, which seriously interferes with the second uranium purification cycle downstream. Furthermore, since this second extraction cycle is to be effected in the oxidized aqueous solution, treatment with an oxidizing agent is necessary. Again, there are attendant difficulties. Thus, if the oxidation is performed with air, the operation is slow and requires additional equipment. If oxidation is effected by means of a chemical oxidant, it involves the introduction of harmful foreign ions; for example, the introduction of chlorate ions results, after reduction, in the formation of chloride ions, which are powerful corroding agents. The use of oxygenated water (another possible oxidant) is expensive.